Conventionally, an electric power steering (EPS) in an automotive steering system has been proposed and become commercially practical. An electric power steering apparatus configured to generate an auxiliary steering torque using an electric motor according to the steering torque applied to a steering wheel and transmit this auxiliary steering torque to a steering shaft is currently under development. In such an electric power steering apparatus, the motor is connected to the steering shaft through a speed-reduction mechanism so that the rotation of the motor will be transmitted to the steering shaft after being decelerated by the speed-reduction mechanism.
Further, attention on autonomous driving technology to steer a vehicle automatically has been focused in recent years, and various techniques have been developed. When automatic steering is realized in an electric power steering apparatus, structures having steering angle control (to calculate and control a motor electric current command value used to make the steering angle of the steering follow a desired angle) and assist control (to calculate and control a motor electric current command value used to apply an auxiliary steering force (assist) to the steering mechanism using the rotational force of the motor) independently to switch between these outputs are adopted (for example, see Patent Documents 1 to 3).